As the largest rare earth mineral reserve in the world, bastnaesite is mostly exploited and utilized in the world. Bastnaesite provides almost 70% of rare earth raw materials, and abundant fluorine resource is contained in it. China, as a country rich in rare earths, has extremely abundant bastnaesite resources. For example, the Bayan Obo Mining District in Inner Mongolia, the Mianning rare earth mining district in Sichuan, the Weishan rare earth mining district in Shandong and the like, are all large rare earth deposits that mainly contain bastnaesite. The production of rare earth in China meets 95% of global demands of the rare earth. Bastnaesite in Sichuan is the second largest rare earth resource in China, and the history of smelting and separation of bastnaesite in Sichuan reaches 20-years. In the 1990s, Baotou mixed rare earth ore processing technology, including an oxidation roasting process and a sulfuric acid leaching process, was introduced in Sichuan; the bastnaesite is smelted to obtain a rare earth sulfate solution containing a fluorine element and a tetravalent cerium, then the obtained rare earth sulfate solution is subjected to double sulfate precipitation, an alkali treatment and dissolved with acid to extract rare earth chlorides rich in cerium and rare earth chlorides containing little cerium. This technology has a long process involving a dozen of solid-liquid separation processes, while the recovery rate of rare earths is only about 70%. An chemical method of oxidation roasting and hydrochloric acid leaching was successfully developed on the basis of the Mountain Pass rare earth mine smelting technology in 2000. The main processes of the chemical method include an oxidation roasting process, a hydrochloric acid leaching process, an alkali decomposition process and a process of leaching cerium using hydrochloric acid, and the chemical method can produce 98% of CeO2 and rare earth chlorides containing little cerium. By using this method, 30˜35% of REO (rare earth oxides) can be directly dissolved from minerals through the oxidation roasting process and the hydrochloric acid leaching process. Rare earth chlorides containing little cerium can be directly obtained, and the total recovery rate of rare earth is up to 93%. A great amount of high-value non-cerium rare earths enter a cerium-rich residue after the bastnaesite is processed using this method, leading to the under-priced use of high-value elements; for this reason, an alkali decomposition step, a washing step, a hydrochloric acid leaching step and the like need to be bridged. The comprehensive utilization of associated fluorine resources has never been realized during the treatment process of the bastnaesite; and the fluorine element enters the cerium-rich residue in the hydrochloric acid leaching process and then mostly enters wastewater along with the implementation of alkali conversion, thus increasing the difficulty of eliminating environmental pollution. Fluorine-containing wastewater is usually processed by using lime or calcium hydroxide; a great amount of calcium fluoride waste residue results from this process, thus increasing the difficulty of subsequent processing; Moreover, the effect of the fluorine removal from the wastewater is limited under a strongly basic condition, and the emission of the fluorine element contained in the wastewater hardly stably meets an emission standard (below 10 ppm), and this has become an environmental issue which hinders the development of rare earth industry in China, especially, in Sichuan.
Changchun Institute Of Applied Chemistry Chinese Academy Of Sciences, General Research Institute for Nonferrous Metals and other research institutions have been making efforts in recent years to develop a green smelting technology to mainly realize the problem of comprehensive utilization of the elements of thorium and fluorine that are generated accompanying an extraction separation flow; according to the green technology, the tetravalent cerium, the thorium and the fluorine all enter a rare earth sulfate solution through an oxidation roasting process and a sulfuric acid leaching process, then the cerium, the thorium and the fluorine and other trivalent rare earth elements are separated and extracted from the rare earth sulfate solution with a one-step or multi-step extraction and separation technology; the core of the green technology is solving the comprehensive utilization of the fluorine and obtaining a pure thorium product by forming a cerium fluoride or a cryolite in the extraction and separation process. Although this technology has been industrially tested, the large-scale promotion and application of the new technology is limited for its high preliminary investment and high production cost, especially for the absence of the demand for the pure thorium,
A bastnaesite decomposition method is disclosed in Chinese Patent Application No. 200610114588.9. According to the application, a bastnaesite and carbonates are mixed in such a proportion that the ratio of non-cerium rare earths contained in the bastnaesite to the cerium contained in carbonate is 1:0.5 to 1:2; the mixture is roasted; a great amount of cerium carbonate is added to solidify the fluorine contained in the bastnaesite, then, the cerium and non-cerium rare earth elements are separated by using an acid leaching method, and finally the fluorine is left in a cerium-rich residue in a solidified state. No fluorine is recovered in this technology. Besides, because of the input of a great amount of roasting promoters of rare earth carbonates, the workload of a mineral roasting processing, an acid dissolution processing, an extraction and separation processing and a rare earth precipitation processing are increased, and the consumption of the acid and the alkali is increased, leading to an increase in production cost.
A method for producing rare earth chlorides containing little cerium and cerium fluoride in one-step is disclosed in Chinese Patent Application 200810046146.4. According to the application, an oxidation roasted rare earth concentrate is selectively dissolved with dilute hydrochloric acid; then a strong oxidant like sulfuric acid, nitric acid, perchloric acid or potassium permanganate is added to perform a catalytic leaching process so that cerium reacts with fluorine ions to generate cerium fluoride, and the generated cerium fluoride enters a residue. This method omits an alkali conversion step, a washing step and a hydrochloric acid dissolution step; and inhibits or eliminates the water pollution that caused by the fluorine by solidifying the fluorine in the residue. However, the addition of the strong oxidant leads to the release of chlorine gas because of the oxidation of hydrochloric acid, resulting in environmental pollution which deteriorates production and operation conditions, a decrease in the amount of effectively used hydrochloric acid and an increase in the consumption of acid.
A method of activating, leaching and decomposing bastnaesite is disclosed in Chinese Patent Application 201010517433.6. According to the application, the bastnaesite is roasted and activated at a temperature below 400° C., and leached using hydrochloric acid and filtered. Then the filtrate is added with rare earth hydroxides to remove impurities, such as iron and thorium. As minerals are not completely decomposed after the bastnaesite is roasted at a low temperature, a leaching rate of rare earth is low, and although the fluoride content of the residue resulting from the leaching process is greater than 95%, the fluorides are all incompletely decomposed bastnaesite. Thus, the residue resulting from the leaching process is subjected to a sodium hydroxide alkali conversion processing, water washing in order to remove fluorine and dissolving with hydrochloric acid so as to recover rare earths; and it is needed to evaporate the fluorine-containing alkali liquid to recover the fluorine element. The whole process involves multiple solid-liquid separations; meanwhile, still using a technology consisting of the hydrochloric acid leaching process, the alkali conversion process and another hydrochloric acid leaching process. Hence, this method has problem of high chemical materials consumption, high energy consumption and high environment protection cost.
Foregoing technologies are advantaged in low investment and relatively low production cost, but disadvantaged in low purity of products and difficulties in recovering thorium and fluorine dispersed in the residue and wastewater, which lead to resources waste and environment pollution. With strengthening the protection of resources and environment, ‘Emission Standards of Pollutants from Rare Earths Industry’ was formulated by Ministry of Environmental production of the People's Republic of China and officially put into action on Oct. 1, 2011. The widely used technology, including a hydrochloric acid leaching process, an alkali conversion process and another hydrochloric acid leaching process can hardly meet the new environment protection requirement set forth in the new emission standards of pollutants from rare earths industry, and enterprises need to increase the investment on the environment protection facilities and the expense on environment protection processing.
Thus, it is urgently to develop a highly operable clean technology, and it is capable of comprehensively recovering the resource of rare earths and fluorine element with a simple technical flow at a low production cost.